It is desirable to automate many industrial or other processes for a number of reasons. For example, the automatic control of the piloting of an airplane is desirable to avoid human fatigue and error over long flights. In many industrial processes such as the operation of a factory, automatic control is preferable to human control since the number of variables and speed of reaction makes human control at times impractical. In other processes, such as the operation of a household furnace, the use of an automatic control is the only practical solution since direct human control would be uneconomic.
In all these control applications, the automatic controller must react to changes in process output operating parameters which in turn will require modification of a process input parameter. Take for example, the household thermostat, the decrease in temperature of the house below a preset temperature causes the thermostat to activate the furnace control. The activation of the furnace eventually causes the temperature in the house to rise beyond the preset temperature, causing the thermostat to deactivate the furnace.
In more complicated systems, a more complex response of the controller may be required. For example, where the initial error in the system is great or has been present for a long time, the response from the controller may be more drastic than where the initial error is slight or recent in time. Similarly, where the error is changing rapidly with time, the response of the controller may be more drastic than if the error is changing slowly. Controllers which are able to react in this way are referred to as proportional-integral-derivative, or PID controllers, and such controllers have dominated industrial controller applications to date. These controllers work by examining the instantaneous error between the process output value and the set point. The proportional term causes a larger control action to be taken for a larger error. The integral term causes a larger control action to be taken if the error has persisted for some time. The derivative term supplements the control action based on the rate of change of the error.
The value of PID terms depend on characteristics of the process and must be tuned accordingly to yield satisfactory control. Properly tuned PID controllers provide adequate control for a large portion of industrial applications. However, there are many processes with time-variant or nonlinear characteristics which are difficult to control with fixed parameter PID controls. It is well known that conventional PID controllers cannot always control to an "ideal" control response. This limitation is inherent in the linear response of the PID controllers to error stimulus. When the characteristics of a SISO process operation are not linear in nature, a PID controller may not provide the desired accuracy. Varying a process control input does not necessarily mean a process output will vary in a linear fashion. A nonlinear control response to an error stimulus can significantly improve control performance with respect to response time, over shoot and control stability.
Therefore, it is an object of the present invention to provide a non-linear control response system for a single input-single output process.